The receptor-mediated adhesion of cells to surfaces under flow is a critical event in many complex physiological processes, such as inflammation, cancer cell metastasis, lymphocyte homing, and hemostasis, and technologies such as stem-cell homing in bone marrow transplantation and intra-vascular targeted drug delivery Proper understanding of the dynamics of adhesion can lead to a molecular understanding of immune cell function and the construction of leukocyte mimetics for targeted drug delivery. Hypothetically, different dynamic states of adhesion - such as rolling and firm adhesion - as well as the detailed dynamics of adhesion, are controlled by functional properties of adhesion receptors, such as their rates of binding to and dissociation from their ligands, and the mechanochemical response of the receptor-ligand bonds. One specific aim of this proposal is to relate the functional properties of individual receptor-ligand bonds to the dynamics of adhesion. For several well-characterized receptor-ligand pairs (antigen-antibody; selectin-carbohydrate; and streptavidin-biotin) and their mutants, the investigators will measure the kinetics of binding and release using fluorescence or surface plasmon resonance; and they will also measure the mechanico-chemical response using either a microcantilever device or biomembrance force probe. Adhesion measurements will be performed in flow channels using the same chemistries using receptor-coated beads, polymeric spheres and cells and well-defined substrates.Compute simulation (Adhesive Dynamics) will be used to recreate the dynamics of motion of the beads or cells as a function of receptor-ligand functional properties, and be used to develop concise relationships between molecular properties and adhesion dynamics. A simple two parameter model of molecular response to force leads to a simple "state diagram" relating molecular properties to adhesion, but more calculations need to be performed to incorporate the more complex dynamic response of molecular recently elucidated with the microcantilever and biomembrane force probes. The innovativeness of this proposal is the multi-level investigation, spanning many molecules and methods, to give a comprehensive, fundamental description of adhesion. A second specific aim of the proposal is to illustrate the usefulness of this fundamental description by constructing and characterizing deformable, porous, biodegradable microspheres (leukocyte mimetics) for targeted drug delivery. These microspheres can be coated with adhesive ligands that code for rolling or firm adhesion , and can be used for the treatment of vascular diseases.